The collection and analysis of microseismic events associated with hydrofracturing a well to improve production or due to production from reservoirs are generally well known. Such microseismic events are essentially small earthquakes (e.g., having a Richter magnitude of less than three) that result from stress changes within the geological structures associated with a well or reservoir. Typically, these stress changes are induced during the extraction or injection of fluids into the well or reservoir. More specifically, the anisotropic nature of earth stresses within a reservoir results in the accumulation of shear stresses on geological structures such as faults, fractures, etc. These accumulated shear stresses are often released during depletion (e.g., extraction processes) and stimulation (e.g., during hydraulic fracture stimulation) operations. The release of these shear stresses results in the emission of acoustic energy or sound that can be detected by devices such as, for example, geophones, accelerometers, etc., and analyzed to determine certain physical characteristics of the well and/or reservoir.
Some past efforts have attempted to analyze microseismic data to optimize well placement and to predict well performance. In particular, some of these efforts have focused on identifying the locations of microseismic events to map fractures to enable the prediction of well performance and/or optimize well placement. For example, microseismic data may be analyzed to determine fracture orientation, extent or size, and estimated growth, all of which are factors that affect optimal well placement and, ultimately, well production or performance. One such effort is described in Society of Petroleum Engineers (SPE) paper number 88695, entitled “Contribution to the Valuation of Microseismic Monitoring Data Recorded from Treatment Well—Results Based on 20 Hydro-fracturing Jobs Recorded From Treatment Well,” by Kaiser et al., the disclosure of which is incorporated by reference herein in its entirety.
Other efforts have focused on using microseismic event data to improve hydraulic fracture stimulation of a reservoir to thereby increase the productivity of the associated well(s). One such effort is described in SPE paper number 91435, entitled “Successful Application of Hydrajet Fracturing on Horizontal Wells Completed in a Thick Shale Reservoir,” by East et al., the disclosure of which is incorporated by reference herein in its entirety.
While the above-noted uses of microseismic data have focused on determining the spatial characteristics of reservoirs (e.g., fracture location, orientation, extent, etc.), still other efforts have attempted to use microseismic event data to estimate reservoir properties such as, for example, porosity, permeability, fluid saturation, stress, seismic velocity, and rock strength. In addition to spatial characteristics, these other reservoir properties may be useful to control fluid extraction from a reservoir and/or to plan production and/or development of fields. An example system that processes microseismic signals to estimate reservoir properties as noted above is described in U.S. Pat. No. 6,947,843, the entire disclosure of which is incorporated by reference herein in its entirety.